Video of a cappella 'Little Drummer Boy' by Pentatonix goes viral

Pentatonix uploaded its version of "Little Drummer Boy" to YouTube a week ago.

As of Tuesday morning, the video had 8.3 million views.

The five-person a cappella group won the third season of NBC's competition series "The Sing-Off" in 2011 and have since become YouTube stars, with more than 2 million subscribers and more than 14 million people having watched their recent viral Daft Punk cover video.